The present invention relates to cooling engines in general and, more particularly, to cooling engines having a free motion, gas-driven, piston actuated displaced.
Traditionally, free displaced, i.e., free piston cooling engines, work well thermodynamically, but lack sufficient reliability over a long period of time for them to be commercially successful against the currently available mechanical driven cooling engines. The problem with a free, gas-driven displaced is controlling the motion of the displaced at the top dead center and the bottom dead center of its cycle. In order to achieve high thermodynamic efficiency, the volumes at top dead center (TDC) and bottom dead center (BDC) should approach zero. With free displaced machines, this objective is very difficult to achieve without collisions taking place between the displaced and cylinder containing the displaced.
U.S. Pat. No. 4,792,346, issued Dec. 20, 1988, for a "Method and Apparatus for Snubbing the Movement of a Free, Gas-Driven Displaced in a Cooling Engine" discloses a method for snubbing displaced movement that utilizes a magnetic repulsion force between the displaced and each end of the cylinder containing the displaced. Two stationary magnets are placed at the ends of the displaced containing cylinder and the displaced itself has two movable magnets attached to the ends of the displaced in such a manner that they act as magnetic springs, i.e., the like magnetic poles of the stationary and movable magnets at one end face each other and, similarly, the like magnetic poles of the stationary and movable magnets at the other end of the displaced and cylinder face each other.
As the displaced approaches one end of the cylinder, the repulsion force of the magnetic force of the magnetic spring stores the kinetic energy of the displaced and prevents a collision from taking place. When the displaced is allowed to move in the other direction, the stored energy is converted back into kinetic energy in the opposite direction. Thus, the displaced is essentially suspended between the two magnetic repulsion forces which prevent collisions between the displaced and the ends of the displaced containing cylinder. The disclosure of U.S. Pat. No. 4,792,346 is incorporated herein by reference.
U.S. Pat. No. 3,991,586, issued Nov. 16, 1976, for "Solenoid Controlled Cold Head for a Cryogenic Cooler" discloses a closed cycle cryogenic cooler, utilizing two solenoids that selectively drive or selectively brake the regenerator-displacer. The physical position of the regenerator-displacer is used to control the actuation of the solenoids. The disclosure of U.S. Pat. No. 3,991,586 is incorporated herein by reference.
In order to achieve maximum cooling efficiency, the pressure/volume diagram ideally should be a perfect rectangle. Stated in terms of the displacer movement, the displacer should commence its movement from TDC when a predetermined pressure differential is reached and should move to BDC without overshooting the BDC position. Similarly, the displacer should be retained at the BDC position until a predetermined pressure differential is reached and then the displacer should move to TDC without overshooting the TDC position.
Application Ser. No. 07/493,474 discloses bi-directional magnetic detents that provide the dual function of snubbing the displacer to limit the amount of overshooting of the TDC and BDC positions and generating a retaining force to keep the displacer at TDC and BDC until a predetermined pressure differential is reached. Although the amount of overshooting of TDC and BDC is significantly limited in this configuration, it should be eliminated entirely.
It is accordingly a general object of the present invention to provide both a method and apparatus for controlling the movement of a free, gas-driven displacer in a cooling engine.
It is a specific object of the invention to utilize both mechanical and magnetic forces to provide the desired controlling action for the free, gas-driven displacer.
It is a further object of the invention to utilize mechanical forces to prevent overshooting and magnetic forces to retain the free, gas-driven displacer until a predetermined pressure differential is reach.
It is a feature of the invention that the method can be practiced and the apparatus constructed utilizing relatively inexpensive and commercially available mechanical and magnetic components.